Allergic diseases are caused by inappropriate immune responses to harmless antigens, and in one form or another, affect 1 in 4 Americans in the United States with a disproportionate increase especially among children with asthma and food-related hypersensitivities. While malleable genetic and environmental components play an important role, immunological aspects associated with cellular and humoral aspects of atopy remain major risk factors underlying the pathogenesis of these diseases and allergic inflammation in general. Among the immune cells implicated in allergic disease, few have received in recent years the level of attention given to dendritic cells (DC) and basophils. DCs are best known for their capacity to act as antigen-presenting cells (APC) that induce immunological responses by presenting antigen to T cells. In mediating this activity, DCs play an important role in both the priming of adaptive immune responses and in the induction of peripheral tolerance. Ancillary studies in mice now indicate that basophils also possess APC capability that is most important in initiating the Th2 responses that are hallmark in allergic diseases. However, it remains unclear in humans whether specific DC subtypes (and/or basophils) preferentially support allergen-reactive Th2 responses, and whether/how these might be altered by immune-based intervention. Thus, three aims are proposed: Aim 1 addresses the general hypothesis that specific DC subtypes induce phenotypically and functionally distinct allergen-reactive T cell responses. In particular, experiments investigate plasmacytoid and myeloid DC (pDC and mDC, respectively) as well as basophils for their potential to differentially induce T cell responses in dust mite allergic subjects. Since new evidence points to basophils playing a role in modulating DC activity rather than acting as APCs, then this hypothesis will also be investigated. Aim 2 addresses the hypothesis that pDC, mDC, and basophils are differentially primed by innate immune stimuli (e.g. TLR ligands) and by specific cytokines (IL-3/IL-33 vs. IFN- ?) to differentially induce allergen-reactive Th2 and/or Th1 cell responses. Finally, Aim 3 addresses the hypothesis that successful Standard Immunotherapy (SIT) alters the capacity of DC subtypes (and basophils) to promote allergen-specific Th2 cell responses. In particular, our preliminary data show that SIT improves TLR9-dependent innate immune responses in DC that are impaired in allergic disease. We further predict that this increased activity will parallel with increases in IgG4 blocking activity, the induction of allergen-specific Tregs, and clinical improvement. Our unique ability to investigate human DC and basophils makes this application highly innovative, and increases the probability that novel, significant, and clinically relevant findings will be discovered.